Engine starting control has a significant impact on engine emissions and engine speed run-up. Fuel amount influences both torque and emissions. Torque is necessary to accelerate an engine from cranking speed up to idle speed. Further, low starting emissions are desirable when catalysts are cold and their efficiency is low. In general, fuel quantities producing air-fuel mixtures rich of stoichiometry increase engine torque while mixtures lean of stoichiometry reduce emissions during a start. Therefore, it is important to provide consistent and accurate fuel amounts to ensure engine speed run-up with reduced emissions.
One method to adjust fuel during a start is described in U.S. Pat. No. 4,432,325. This method provides for a metered fuel quantity maintained at a first constant value during a first phase of an engine starting period, the metered fuel quantity is changed to a threshold in accordance with an engine variable during the second phase of the engine starting period, and the metered fuel quantity is maintained at said threshold value during the third phase of said engine starting phase.
The inventors herein have recognized a disadvantage with this approach. That is, the approach neglects changing engine air amounts produced by engine pumping and changing engine speed. In other words, when an engine starts to turn it begins to pump down the intake manifold producing different air amounts in each cylinder during a start. When different cylinder air amounts are present with constant fueling, different in cylinder air-fuel ratios can be produced. This is an undesirable condition in terms of engine emissions and torque generation.
Another method to adjust fuel while an engine is cold is described in U.S. Pat. No. 5,870,986. This method provides an injection number counter means for counting a total number obtained by summing the fuel injection number in all cylinders from a start of an operation of the internal combustion engine; and a fuel injection timing changing means for advancing by a predetermined period a fuel injection start timing. When the counted value of said injection number counter means is less than a predetermined value, the fuel injection timing changing means sets the fuel injection start timing at a timing in synchronism with the intake stroke start timing of each cylinder. In the case where the above-described value of said injection number counter means is equal to or more than a predetermined value, the fuel injection start timing is set at a timing earlier than the intake stroke start timing of each cylinder.
The inventors herein have recognized a disadvantage of this approach as well. Namely, the approach injects fuel on an open valve in order to avoid having to compensate for fuel that attaches to engine surfaces when injecting fuel in a cold engine on a closed valve. However, injecting fuel on an open valve in a cold engine poses other issues. Specifically, open valve injection during a start may expose spark plug tips to raw fuel, potentially impeding the plugs from firing, raising engine emissions and producing inconsistent engine speed run-up or even preventing the engine from starting. In addition, open valve injection during a start may expose cylinder walls to injected fuel, washing cylinder walls with fuel, breaking down oil film, and causing scratches in the cylinder.